1. Field of the Invention
This invention is directed to devices for detecting cracks, flaws and other defects in a layered structure. More specifically, the devices utilize eddy currents that are generated within the layers of the structure for the detection of the cracks, flaws and other defects in the layers of the structure.
2. Description of Related Art
Various instruments for flaw detection in structures have been developed that rely upon the generation of eddy currents in the body of the structure being tested. Flaws in the metallic structures are detected by their perturbation of the eddy currents. The eddy currents are generated in the metallic structures by positioning the structures within an alternating magnetic field. Perturbation of the eddy currents because of the localized presence of a defect in the metallic structure creates a resultant change in the magnetic flux associated with these eddy currents. When such change is detected, it is an indication of the presence of defects in the structure. Such use of eddy currents has been practiced for detecting flaws in solid metallic slabs, metallic pipes, and in layered metallic structures such as the outside surfaces of aircraft.
U.S. Pat. No. 3,437,918 to Arnelo describes an eddy current detection system for a slab structure. Further, eddy current detection systems for slabs are described in U.S. Pat. No. 4,534,405 to Hulek, et al., U.S.S.R. patent 1,155,930, Japanese patent 60-147648, and Japanese patent 59-162448. Eddy current detection for solid pipe like structures is disclosed in U.S. Pat. No. 3,694,740 to Bergstrand, et al., Japanese patent 61-14569, British patent 631,987 to Rudorff, and U.S. Pat. No. 4,855,677 to Clark, et al. Detection systems for layered structures are found in U.S. Pat. Nos. 4,219,774 to Rogel, et al., 4,414,508 to Davis, et al., 4,495,466 to Lakin, et al., 4,496,904 to Harrison, 4,445,089 to Harrison, 4,677,379 to Arnaud, et al., and 4,095,181 to Harris et al.
Many of the above patents describe systems wherein a single coil is utilized to induce a magnetic field in a test piece. Flaws are detected by noting changes in the impedance of the test coil. In other of the above patents, both a drive coil and a sense coil are utilized. Generally, detection of flaws is achieved utilizing voltage variations in the sense coil. The voltage variations are detected utilizing a null bridge.
In the above noted Clark patent, U.S. Pat. No. 4,855,677, a plurality of coaxial coils, each driven at a different frequency, is utilized to detect flaws at different depths in pipes. In the above noted Japanese patent 59-162448, the sense elements, which are independent of the drive coil, are arranged in two sets along two radii which are 180.degree. apart. The sets of sense coils are rotated directly underneath the center of the drive coil.
As is described in the noted Lakin patent, U.S. Pat. No. 4,495,466, the skins on wings and the body of aircraft are constructed by fastening several layers together with a large number of fasteners. Each of the fasteners is positioned in a hole that passes through each of the layers. Fatigue cracks about the fastener holes develop in response to stress of the aircraft structures.
In aircraft structures, if these cracks are detected when they are small, the fastener having the crack adjacent thereto can be removed, the hole drilled out to a slightly larger diameter that includes the crack within its bounds, and a larger fastener inserted. This thus removes the crack and, in doing so, eliminates a structural defect without severely compromising the integrity of the part that initially bore the defect.
For aircraft structures or other structures, the above noted U.S. Pat. No. 4,219,774 to Rogel and U.S. Pat. No. 4,414,508 to Davis require removal of the fastener for inspection of the fastener holes. This is a time consuming and laborious process. Further, aside from the time and expense, in the process of removing the fasteners new flaws can be introduced into the layered structure.
In order to sense defects in deeper layers, as for instance a second layer of a two layer system, expedients have been devised for separating signal from the first layer from that of the second layer. Such expedients include multiple frequency sensing as discussed in the above noted U.S. Pat. No. 4,495,466 to Lakin, or sensor movement as is common in above noted U.S. Pat. Nos. 4,095,181 to Harris, 4,445,089 and 4,496,904 to Harrison, and 4,677,379 to Arnaud. While the methods practiced in these patents have resulted in improvements over older methods, flaw detection is still a difficult and time consuming process.
Composite materials have been increasingly used in aircraft construction, especially military aircraft. Such composite materials serve to shield detection of flaws in the deeper, underlying metallic structures on which the composite materials are fastened. Additionally, the underlying metallic structures tend to be of complex shape that can distort detection systems such as that of the above noted Arnaud U.S. Pat. No. 4,677,379. This arises because the Arnaud patent relies upon uniform probe movement along uniformly spaced rivet arrays in essentially flat uniform structures. Indeed, as is noted in that patent, the sense coils are stated to be separated from the primary windings at a distance that is equal to one-half of the distance separating consecutive rivets in a succession of rivets. This requires that the pattern of the fasteners be very uniform. Such a uniform pattern may or may not be used in those constructions wherein composite materials are fastened to underlying metallic structures.
Quadrature detection is mentioned in both the above noted U.S. Pat. Nos. 4,677,379 to Arnaud, et al. and 4,496,904 to Harrison. Such a detection technique utilizes both a magnitude and a phase component of a signal for analysis of that signal. As noted above, however, the Arnaud patent requires the probe geometry to be related to the fastener geometry, and the Harrison U.S. Pat. No. 4,496,904 patent requires the probe to be rotated directly over the center of the fastener for defect sensing.
And, in an NBS Staff Report for February of 1980, a publication of the National Bureau of Standards of the U.S. Department of Commerce, an eddy current imaging system is described which is said to provide information about the type and shape of surface flaws as well as limited data on sub-surface flaws, but which provides no capability to account for sub-surface structural geometries. Additionally, orthogonal pairs of drive coils on opposing faces of a square array are operated simultaneously with signals 180.degree. out of phase with each other.
Other techniques that have been utilized for the inspection of aircraft structures include radiographic methods. Such radiographic methods, however, miss up to 75% of the cracks because there is a lack of density differences between a part having a small crack and a part not having a crack therein. Further, such detection methods are difficult to implement in many areas of aircraft because the geometry of the aircraft structure prevents placing an x-ray film on one side of the structure and a suitable device for generating x-rays on the other.